A digital signal process system in a mobile communication base station processes a signal to be input to a modem by including a Digital Down Converter (DDC) for converting an input digital signal to a baseband signal and an Automatic Gain Control (AGC) for converting the power of the converted baseband signal to a constant value and outputting the same. Here, the DDC generally maintains a high resolution in order to prevent distortion of a signal and a high Signal to Noise Ratio (SNR) characteristic. However, since in reality data required by a modem for signal demodulation has a resolution lower than that of output data of the DDC, a digital signal process system extracts only data corresponding to an effective bit from the output data of the DDC and provides the same to the modem via the AGC. Therefore, the AGC performs a function of controlling the gain of a signal so that the power of an output signal may always maintain a predetermined level regardless of power change of an input signal.
FIG. 1A illustrates a relationship between a data signal output from a DDC and input to an AGC and an effective bit, and FIG. 1B illustrates a relationship between an output data signal of an AGC and an effective bit. Here, assuming that the output effective bit of the AGC is 11 bits, the range of a value that can be represented by the effective bit is 1023 to −1024. As illustrated in FIG. 1A, in the example where the peak power of a data signal input to the AGC is about 1700, since an input signal is a signal not included in the effective bit range, the AGC may control the magnitude of the input signal whose peak power is about 1700 and convert the signal to a signal included in the effective bit range (1023 to −1024) and output the same as illustrated in FIG. 1B.
The conventional AGC uses a method of controlling an output signal using, average power. That is, the conventional AGC calculates average power of an entire input signal and controls the magnitude of an output signal based on the calculated average power. Since this method using the average power has a very long period for calculating and reflecting the average power, when the power of an input signal changes fast instantaneously, it is difficult to recognize the instantaneous power change, so that it is difficult to normally control the magnitude of an output signal.
Also, in the method using the average power, when the power of an input signal drastically changes, the speed of calculating the average power of the power change of the input signal is delayed, so that the signal is lost consequently, and a clipping error may occur.